`RlsPicker.pickSubchannel()` does not run in SynchronizationContext, but it calls `CachingRlsLbClient.get()` which assumed running in SynchronizationContext. Fixed by removing `synchronizationContext.throwIfNotInThisSynchronizationContext()`. `CachingRlsLbClient.get()` is actually thread-safe in the sense it's guarded by lock, and `DataCacheEntry`'s fields are final.
`ChildPolicyWrapper.picker` was not thread-safe. Fixed by making it volatile.
Changed the test a bit since the old test doesn't really test things well.
- Use gradle configuration `api` for dependencies that are part of grpc public api signatures.
- Replace deprecated gradle configurations `compile`, `testCompile`, `runtime` and `testRuntime`.
- With minimal change in dependencies: If we need dep X and Y to compile our code, and if X transitively depends on Y, then our build would still pass even if we only include X as `compile`/`implementation` dependency for our project. Ideally we should include both X and Y explicitly as `implementation` dependency for our project, but in this PR we don't add the missing Y if it is previously missing.